The present invention relates in general to a method and an apparatus for transmitting data, and more particularly to a method and an apparatus for transmitting data employing an OFDM (Orthogonal Frequency Division Multiplex) system, and a method and an apparatus for receiving an OFDM signal.
In recent years, as for the modulation method which is suitable for being applied to the digital audio broadcasting for mobiles and terrestrial digital television broadcasting, the OFDM system which features a robustness to multi path fading and ghost has received much attention. The OFDM system is one of the multi-carrier modulation systems and is a transmission method of subjecting n carriers (n is in the range of several tens to several hundreds) which are authogonal to one another.
Then, as shown in FIG. 2, the modulated signal is transmitted which is acquired by adding a large number of digital modulated waves to one another and by subjecting the I-axis and the Q-axis to the orthogonal modulation. While as for the above-mentioned digital modulation method, in general, the 4 DQPSK (4 Differential Quadrature Phase Shift Keying) system is often employed, it is also possible to employ the multi-value modulation system such as the 16 QAM (16 Quadrature Amplitude Modulation) or the 32 QAM.
In addition, as shown in FIG. 3, the symbol of the OFDM is constructed such that the guard interval for reducing the influence of the delayed wave is added to the effective data symbol. The guard interval is the signal which is added cyclically to the signal of the effective data symbol. The guard interval is described in, for example, U.S. Ser. No. 09/099390 (corresponding to European Patent application No. 98111075.2 filed on Jun. 17, 1998) entitled “OFDM MODULATOR AND OFDM MODULATION METHOD FOR DIGITAL MODULATED WAVE HAVING GUARD INTERVAL”, the disclosure of which is hereby incorporated herein by reference. By adding the guard interval, degradation due to the inter-symbol interference thereof can be avoided against the delayed wave generated due to the delay time within the guard, and hence the OFDM system is robust to multi path fading.
On the other hand, since in the OFDM system, the frequency interval between the carriers is narrow, interference between the carriers due to the carrier frequency error between the transmitter and the receiver and due to the sampling clock frequency error of the demodulation system may readily occur, and hence high accuracy is required for those frequencies.
For this reason, in order that the receiver may continue to receive properly the OFDM signal, there is required the processing of recovering the sampling clock with which the sampling clock frequency of the receiver is made coincide with the sampling clock frequency of the transmission signal. In addition, in the case where the frame period and the symbol period of the received signal fluctuate with time, the sampling clock frequency of the receiver needs to follow that fluctuation.
For this reason, the transmission side transmits the OFDM transmission signal in which the transmission frame is constituted by the effective data symbol and the several kinds of synchronous symbol groups. An example of the constitution of the transmission frame is shown in FIG. 4.
The reception side executes the sync pull-in processing on the basis of the synchronous symbol. Then, the sampling clock frequency on the transmission side is synchronized with the sampling clock frequency on the reception side to demodulate the OFDM signal.
In this connection, a prior art technique relating to sync pull-in based on the synchronous symbol or the like is disclosed in JP-A-7-321762 for example.